1. Field of the Invention
The present invention relates to a method for controlling a positioning device, a positioning device, and a lithographic apparatus provided with a positioning device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
A lithographic apparatus includes various moving parts that are positioned using at least one positioning device such as a linear or rotating motor. In a lithographic apparatus, examples of moving parts are a wafer stage, a reticle stage, a handler (robot arm), etc. A wafer stage may include different positioning devices for moving a wafer support in multiple degrees of freedom to desired positions with a desired speed, acceleration, etc. Likewise, a reticle stage may include different positioning devices for moving a reticle support in multiple degrees of freedom to desired positions with a desired speed, acceleration, etc.
A positioning device includes a first part, called a stator, which is stationary relative to a predefined frame of reference, and further includes a second part, called a mover, which moves relative to the stator through the generation of electromagnetic forces between the stator and the mover. Positioning the mover relative to the stator is performed using a controller. The controller may include a control characteristic being any combination of a proportional (P) control function, an integrating (I) control function, and a differentiating (D) control function.
For controlling the actuator by the controller, the controller receives a position signal from a position sensor detecting the position of the mover relative to the stator. The position signal is compared with a setpoint signal defining the desired position of the mover relative to the stator. The difference between the position signal and the setpoint signal is obtained as one or more error signals, and on the basis of the one or more error signals, the controller generates positioning device control signals for controlling the position and movement of the positioning device such as to reduce or eliminate position errors. This method of control is also referred to as feedback control, since the actual mover position is fed back to a controller input to generate the one or more error signals. The feedback control forms a closed loop.
In the process of generating positioning device control signals, the controller shows a control characteristic, which is a feature of the controller. The control characteristic is the way in which the controller operates in response to detecting position errors.
During motion of the mover of the positioning device, such as motion of the mover of a wafer stage or a reticle stage, low-frequency disturbances limit the closed-loop performance. Under linear feedback, additional low-frequency disturbance rejection can improve upon this performance, for example by increasing the integrator control function gain. A problem, however, is that the settling behavior at the start of a constant velocity range is deteriorated by increasing the integrator control function gain.
Generally, a problem in adequately handling time-varying disturbances is that in a certain time period a controller with a specific control characteristic related to the disturbance spectrum in this time period is required, whereas in another time period this spectrum generally changes, and therefore a different control characteristic is required.
In the state of the art, it has been proposed to change the gain of the controller depending on the type of disturbance experienced. However, changing the control characteristic to obtain a better handling of disturbances having one disturbance spectrum may seriously affect the controller's capability to handle a disturbance of another kind.